Electrochemical processing of inorganic chemicals in electrolytic diaphragm cells for the production of other inorganic materials is well known. The electrolytic cell generally comprises an anolyte compartment containing an anode, a catholyte compartment containing a cathode, and a microporous diaphragm that separates the anolyte compartment from the catholyte compartment. Diaphragms are used, for example, to separate an oxidizing electrolyte from a reducing electrolyte, a concentrated electrolyte from a dilute electrolyte, or a basic electrolyte from an acidic electrolyte.
A non-limiting example of a diaphragm electrolytic cell is the electrolytic cell that is used for the electrolysis of aqueous alkali metal halide solutions (brine). In such an electrolytic cell, the diaphragm is generally formed on the cathode and separates an acidic liquid anolyte from an alkaline catholyte liquor. The electrolysis of alkali metal brine generally involves introducing liquid brine into the anolyte compartment of the cell and allowing the brine to percolate through the brine-permeable microporous diaphragm into the catholyte compartment. The microporous diaphragm is sufficiently porous to allow the hydrodynamic flow of brine through it, while at the same time inhibiting the back migration of hydroxyl ions from the catholyte compartment into the anolyte
compartment. When direct current is applied to the cell, halogen gas is evolved at the anode, hydrogen gas is evolved at the cathode, and an aqueous alkali metal hydroxide solution is formed in the catholyte compartment. In the case of aqueous sodium chloride solutions, the halogen produced is chlorine and the alkali metal hydroxide formed is sodium hydroxide. Catholyte liquor comprising alkali metal hydroxide and unconverted brine is removed from the catholyte compartment of the cell.
During electrolysis, it is not unusual for the diaphragm of a diaphragm electrolytic cell to allow too high a flow of liquid anolyte into the catholyte compartment, e.g., by developing perforations (holes) in the diaphragm. When the flow of liquid anolyte is too high, the concentration of the principal product formed in the catholyte compartment is lowered, which results in increased costs for unit operations employed to work-up and purify that product, as well as an increase in the amount and cost of recycling process streams from those unit operations. In the case of diaphragm chlor-alkali electrolytic cells, too high a flow of brine through the diaphragm is evidenced by lower than desired concentrations of alkali metal hydroxide and higher than desired concentrations of hypochlorite ion in the catholyte liquor. When such a condition exists, there is a need for means to lower the flow of anolyte through the diaphragm, e.g., through perforations that may have developed in the diaphragm during electrolysis.